Light-emitting devices or apparatuses such as optical semiconductor devices are practically used typically in indoor or outdoor display boards, light sources for image reading, traffic signals, and large-screen displays, in which emitters and surroundings thereof are protected by sealing mainly with epoxy resins. Exemplary epoxy resins which have been widely used as the sealants (encapsulants) include aromatic epoxy resins having a bisphenol-A skeleton.
Recent technical advances allow light emitting diode (LED) displays to have dramatically higher outputs and to emit further bluer or whiter light. This allows light emission with high output at relatively short wavelengths. When the aromatic epoxy resins, however, are used for sealing of the light emitting diode displays, their aromatic rings absorb light with such a short wavelength, and this causes the resins to deteriorate and to turn to yellow and thereby causes luminance degradation and discoloration.
Known as epoxy resins having satisfactory transparency and thermal stability are liquid alicyclic epoxy resins having an alicyclic skeleton such as 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate, an adduct of 3,4-epoxycyclohexylmethyl 3′,4′-epoxycyclohexanecarboxylate and ε-caprolactone, or 1,2,8,9-diepoxylimonene. Cured articles obtained from these alicyclic epoxy resins, however, are susceptible to various stress and are poor in thermal shock resistance (cracking resistance) so that they are, for example, susceptible to cracking as a result of repeated temperature cycles (repeating heating and cooling procedures). Accordingly, they hardly ensure reliability typically of resulting electronic components.
As a possible solution to improve cracking resistance, there is known the use of an epoxy resin composition containing diglycidyl ether of nucleus-hydrogenated bisphenol-A for sealing of optical semiconductor devices (Patent Literature (PTL) 1). However, a cured article obtained from the epoxy resin composition has problems typically in coloring, weather resistance, and thermal stability. PTL 2 describes a technique for imparting high toughness to an epoxy resin by dispersing a core-shell polymer in the epoxy resin. However, this literature does not make mention of the transparency of the cured article obtained from the epoxy resin. Independently, there is known a technique of compounding a polyether polyol in an epoxy resin, and dispersing particles in the epoxy resin, which particles have a core structure composed of a butadiene rubber, and a shell structure composed of a methyl methacrylate resin (PTL 3). However, the cured article therefrom is not satisfactory in transparency, because the particles used in the resin have a butadiene rubber core structure. Specifically, under present circumstances, there is found no optical semiconductor sealing resin composition which gives a cured article that can exhibit excellent cracking resistance while maintaining satisfactory thermal stability and high transparency.